The Silicon Photomultiplier (SiPM) is multipixel (multi-SPAD) silicon photodiode with a number up to a few thousand independent micropixels (with typical size of 10-100 um) joined together on common substrate and working on common load. Each pixel detects the photoelectrons with a gain of about 106. Solid state photomultipliers (SSPMs), which are also commonly referred to as MicroPixel Photon Counters (MPPC) or MicroPixel Avalanche Photodiodes (MAPD) have become popular for use as photosensors. For example, SSPMs have been employed in scintillator based nuclear detectors. Typically, SSPMs are implemented as Silicon Photomultipliers (SiPM).
The gain of an SSPM operating in Geiger mode is proportional to the overvoltage (Vov), e.g., the difference between the applied bias voltage (Vbias) and breakdown voltage (Vbr). The Geiger mode process typically results in a well defined single photo electron (SPE) pulses, which can be used to identify breakdown voltage Vbr and the capacitance (Cu-cell) of individual microcells. Conventional approaches have typically utilized intrinsic dark counts or very short, low intensity light pulses (e.g., with an average of 0.1 to 1 detected photon) to detect SPE pulses. That is, the intensity of light pulses is typically specified to trigger on average 0.1 to 1 u-cell per pulse. These conventional approaches work well for small devices (e.g., ˜1 mm2) and when the dark count rate is small. However, for many SSPM-based detector applications, e.g. a PET scanner, larger SSPMs are constructed (e.g., with a typical size of 10-100 mm2).
While SSPMs have a good single photoelectron pulse (SPE) response to low intensity light that can be used for absolute calibration and defining the breakdown voltage, it becomes increasingly difficult to measure SPE pulses as the size of an SSPM increases. One reason for this can be attributed to an increase in dark counts as the size of an SSPM increases. Moreover, conventional approaches to SPE calibration are typically implemented with a high gain low noise amplifier, which can prohibit use readout electronics designed for detection of relatively large signals from scintillators. Therefore, conventional SPE calibration approaches are generally unavailable for calibrating a fully assembled SSPM-based detector using the detectors readout electronics. Even if an SSPM-based detector includes an application specific integrated circuit (ASIC) with a special high gain mode for the calibration, the high dark count rate can still present a challenge to use of SPE pulses to calibrate large area SSPMs.